The present invention relates to a semiconductor device of a non-punch-through type, and more particularly to a reliable semiconductor device which can suppress a fluctuation in ON voltage.
IGBTs (Integrated Gate Bipolar Transistors) are classified into a punch-through type and a non-punch-through type. FIG. 1 is a cross-sectional view of a conventional non-punch-through type IGBT of a withstand voltage of 1700 V.
The conventional non-punch-through type IGBT has an N-type drain region (substrate) 2 and a P-type anode region 4 formed on a first main surface of the drain region 2 by impurity diffusion or the like. A P-type base region 6 is formed in the N-type drain region 2, and further an N-type source region 8 is formed in the region 6. The regions 6 and 8 are also formed by impurity diffusion or the like.
A thin oxide film 10 is formed on a second main surface of the drain region 2. A polysilicon gate electrode 12 is formed on the thin oxide film 10. A metal gate electrode 14 is formed in contact with the polysilicon gate electrode 12. A source metal electrode 16 is formed on the source region 8 and the base region 6 so as to short-circuit them.
A metal anode electrode 18 is formed in contact with the P-type anode region 4. To suppress injection of holes from the P-type anode region 4, in consideration of the structure of the element region, the thinner the diffusion into the anode region 4, the better the characteristic of the IGBT.
The IGBT as shown in FIG. 1, however, is easily affected by the impurity of the surface region. When the IGBT element is kept at a high temperature for a long period of time, Fe contained in the metal electrode is diffused into the substrate. As result, Fe ions and B ions are coupled in a region of the P-type anode region 4 of a low concentration, forming FeB in this region.
FIG. 2 is a graph showing the relationship between the temperature and the ratio of the concentration of Fe ions to that of B ions, disclosed in J. Appl. Phys., Vol. 67, No. 11, Jun. 1, 1990, "A fast, preparation-free method to detect iron in silicon". In the graph, the increase of the value of Fe.sub.1 /Fe means the increase of the amount of FeB.
As shown in FIG. 2, since the amount of FeB in the P-type anode region 4 increases with the rise in temperature, the energy level is changed. Consequently, the lifetime of carriers is also changed, resulting in a problem that stable element characteristics cannot be obtained.
To solve the above problem, the anode region 4 may be formed thick, and the surface portion of the region may have a concentration enough to form an ohmic contact. In this case, however, the amount of holes injected from the anode region into the substrate is increased. As a result, the element characteristics may be considerably changed.